In this article, We are going to perform How to Configure Sendmail to Relay Emails using Gmail SMTP on Ubuntu 18.04/16.04 LTS System.

Introduction

Sendmail is open source Mail Transfer Agent (MTA) used to transfer emails among hosts using Simple Mail Transfer Protocol (SMTP). It is widely used to send email using command line.Here we are configuring to transfer email using gmail smtp.

Prerequisites

• Ubuntu 18.04/16.04 LTS with Minimal Installation
• SSH Access with sudo privileges
• Open Firewall Port: 587

#1: Remove/uninstall/stop postfix

Postfix is the default Mail Transfer Agent (MTA) in ubuntu. First, we have to remove postfix from ubunut, enter below commands to uninstall postfix.

sudo apt-get remove postfix
sudo apt-get purge postfix

#2: Install Sendmail on Ubuntu 18.04/16.04 LTS

First switch to root user.

sudo su

update the system database before installing any packages.

apt-get update

Enter below command to install sendmail and other mail utilities if not installed.

apt-get install sendmail sendmail-bin mailutils

#3: Configure Sendmail to Relay Emails using Gmail SMTP

1. Create Authentication File

We have to create new directory to store gmail account details.

mkdir -m 700 /etc/mail/authinfo

In above directory create a file named gmail-smtp-authentication and input your gmail account here.

nano /etc/mail/authinfo/gmail-smtp-authentication

AuthInfo: "U:root" "I:[email protected]" "P:GMAIL_USER_PASSWORD"

Replace your USER and GMAIL_USER_PASSWORD with your Gmail email id and password respectively.

2. Create Sendmail-Gmail Authentication Database HASH Map

We have to create database map from gmail-smtp-authentication file using makemap utility as below command.

makemap hash /etc/mail/authinfo/gmail-smtp-authentication < /etc/mail/authinfo/gmail-smtp-authentication

This will create authentication database, /etc/mail/authinfo/gmail-smtp-authentication.db

3. Configure Sendmail SMART HOST

SMART HOST is the mail relay which allows SMTP server to route email to among hosts rather than sending to recipient  server. Here we are using smtp.gmail.com on port 587 to relay emails.

Note the below line;

FEATURE(authinfo',hash -o /etc/mail/authinfo/gmail-smtp-authentication.db')dnl

Replace it with your Gmail Authentication sendmail database map you generated above.

Next, Copy and Paste below lines in /etc/mail/sendmail.mc file above MAILER_DEFINITIONS as shown below,

nano /etc/mail/sendmail.mc

dnl #
dnl # Defining Gmail Smarthost for sendmail
define(`SMART_HOST',`[smtp.gmail.com]')dnl
define(`RELAY_MAILER_ARGS', `TCP $h 587')dnl
define(`ESMTP_MAILER_ARGS', `TCP $h 587')dnl
define(`confAUTH_OPTIONS', `A p')dnl
TRUST_AUTH_MECH(`EXTERNAL DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl
define(`confAUTH_MECHANISMS', `EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl
FEATURE(`authinfo',`hash -o /etc/mail/authinfo/gmail-smtp-authentication.db')dnl

dnl #
dnl # Default Mailer setup
MAILER_DEFINITIONS
MAILER(`local')dnl
MAILER(`smtp')dnl

make -C /etc/mail

Output:

make: Entering directory '/etc/mail'
Updating databases ...
Reading configuration from /etc/mail/sendmail.conf.
Validating configuration.
Creating /etc/mail/databases...
Updating auth ...
sasl2-bin not installed, not configuring sendmail support.

To enable sendmail SASL2 support at a later date, invoke "/usr/share/sendmail/update_auth"

Creating /etc/mail/relay-domains
# Optional file...
Updating Makefile ...
Reading configuration from /etc/mail/sendmail.conf.
Validating configuration.
Creating /etc/mail/Makefile...
Updating sendmail.cf ...
The following file(s) have changed:
/etc/mail/relay-domains /etc/mail/sendmail.cf
** ** You should issue `/etc/init.d/sendmail reload` ** **
make: Leaving directory '/etc/mail'

/etc/init.d/sendmail reload

#3: Testing Sendmail Gmail Relay

echo "test mail for sendmail gmail relay" | mail -s "Sendmail Test" [email protected]

sudo /etc/init.d/sendmail start

sudo /etc/init.d/sendmail restart

sudo /etc/init.d/sendmail stop